I'm 15 years old and my penis is at least be 8 inches when I get to my full length. Right now, without being erected my penis length seems to be about 4 inches. Is that normal? When I look down at my penis, it seems to look smaller than what girls tell me after we have sex. However, is there anyway to make my penis bigger without using any pills, oils, or some type of machine?
There are two colloquialisms for what you're asking about.
When a person has a penis that looks about the same size flaccid (soft) that it does erect, folks colloquially call that person a "shower," because their penis shows itself to be pretty much the same as it is erect. When a person has a penis which looks smaller flaccid that it is erect, folks call that kind of a person a "grower," because the growth of their penis with an erection makes a very visible change. Having a penis that does either of those things is normal, as is having a penis which is somewhere in between.
There's no reason to try and change that, and also no way you can change that. There's no reason for a penis to be or look a certain size, erect or not erect. A penis which is 3 inches erect can be just as functional as one which is 8 inches, and what size a penis is flaccid really has nothing to do with anything when it comes to function, sexually or reproductively. You're very infrequently going to meet a person with a vagina who decides to date you or take you home after eyeballing your pants when they meet you and making a determination as to what your penis size is.
Some of that is simple emotional maturity on the part of those people, and some of that is because what size a guy's penis is makes little difference to most people with vaginas. A majority of people with vaginas don't orgasm through intercourse , and some don't even like it very much, no mater what size penis a partner has. For those who do enjoy intercourse or reach orgasm that way, that's got a lot less to do with the size of a partner's penis than both what they do with it and -- far more so -- what they do with their whole body and the other person's whole body. Again, only a minority of people with vaginas are going to reach orgasm from intercourse alone or like intercourse all that much if it's the only thing going on. Sure, you might someday meet what some people call a "size queen," in your life, a person who is very concerned about the size of their partners penis -- and that person may prefer larger penises, or may prefer smaller ones; may prefer those curved upwards, or those downwards -- but sparing folks who are treating partners like sex toys rather than people, even those with those preferences will often not make the selection of their sex partners about nothing but that, or be looking at your crotch on a date to determine how they feel about you. Those folks will just be particularly jazzed when a partner's penis happens to also meet their preferences, and even then, they're the rarity, not the norm.
Who penis size really seems to matter to is men (and not all men are like this: plenty grow out of that fixation), and that tends to have a lot more to do with male posturing and ideas about sexual competition among men than it has ever had anything to do with women. Most women are just going to scratch their heads or toss up their hands when they hear about guys getting all loopy and juvenile about the size of their penises.
If you're looking for something to fix, what I'd suggest is just adjusting the way you think about penises and size issues and toss any concern you've got about it in the rubbish bin. It's not going to benefit you, help you in the sack, benefit your sexual partners, or do anything other than...well, make you look like you're childish. If a sexual partner wants their partner to be fixated on anything, it's on them and on the mutual pleasure of both of you, not on the size or shape of either person's genitals .
Here are a couple more links to get you informed:
